The present invention relates generally to fluid actuators, and more particularly the invention pertains to a piston type fluid actuator having an externally mounted constant force spring return mechanism for insuring fail-safe positioning in case in fluid pressure.
Hydraulic or pneumatic actuators are employed to control operation of numerous types of devices such as valves, jacks, elevators, and heavy machinery. In simplest form, it comprises a cylinder with an open bore in which a piston moves axially when fluid pressure is applied to at least one end of the piston. In case of a failure in fluid pressure, mechanisms are provided which will cause the piston to move the device to a predetermined fail-safe position.
Such mechanisms typically include one or more mechanical coil or constant force springs acting against the piston to move it in the proper direction. A drawback of coil springs is that, due to its spring rate, the applied force against the piston diminishes as it reaches the critical fail-safe position, whereas a constant force spring is coiled in its relaxed state and as it is extended substantially the same force is applied throughout the travel of the piston. For instance U.S. Pat. No. 3,767,160 to Robert F. McCollum discloses a piston-operated regulator valve with constant force springs which urge the valve to a normally closed or open position when there is no fluid pressure. In one design the springs and piston are mounted to the valve body on opposite sides. The disadvantage of this design is it requires separate mounting flanges and seals on both sides of the valve body. In another design the springs are installed inside the piston cylinder. This requires the piston cylinder to be disassembled in order to inspect, adjust or replace the springs. In addition, the number of springs that can be installed in the cylinder is limited by size.
Accordingly, it is an object of the present invention to provide a fail-safe actuator with fail-safe spring return which can be easily mounted on a controlled device and adjusted for spring force.
Another object of the invention is to provide a piston-type fluid actuator having an external constant force spring return mechanism which is readily accessible for inspection, adjustment and replacement of the mechanism without disassembling the actuator or removing it from service.
Still another object of the invention is to provide a piston type fluid actuator in which the number of constant force springs required for fail-safe operation is not limited by the size of the piston cylinder.
A further object of the invention is to provide a lightweight self-adjusting enclosure for protecting a spring return mechanism mounted externally on a piston type fluid actuator.
A further object of the invention is to provide a fail-safe actuator which is less costly to manufacture and maintain than prior designs.
A still further object of the invention is to provide a fail-safe return mechanism utilizing constant-force springs which can be can be easily replaced.
These and other objects and advantages of the invention are accomplished by a fail-safe fluid actuator comprising a piston slidably disposed within a cylinder. In the event of a pressure loss, the piston is urged to a fail-safe position by a plurality of externally mounted constant force spring mechanism. In one embodiment the mechanism is located at one end of the cylinder with a telescoping cover for protecting it from exposure and damage. Another embodiment the mechanism is located at the side of the cylinder whereby the overall length of the actuator is significantly shortened. The fail-safe force applied to the piston by the springs is externally adjustable, and the springs are readily accessible for replacement or repair.